Coupling a distiller and an electrochemical cell for energy production from low temperature heat sources .

We aim at exploiting the heat sources at very low temperature, below 150◦ (e.g. low-concentration solar and industrial waste heat) for production of electrical energy, in particular for small power applications. In this case, traditional techniques, such as organic Rankine cycle, Stirling engines and solid-state devices based on Seebeck effect, are not economically viable. We explore a low-cost, easily downscalable electrochemical method working with a closed cycle: a salinitygradient-power device produces electrical current by consuming the concentration difference between two solutions; the mixed solutions it produces are sent to a distiller which restores the concentration difference, in turn exploiting the low-temperature heat source. The “battery mixing”, a newly introduced salinity-gradient-power technique, enables the exploitation of a wide range of solutes and solvents. In this manuscript, we theoretically analyse the whole cycle, in order to enable an educated choice of solutes and solvents with respect to the efficiency in the conversion of heat into electrical power. We find that the main requirement is a high boiling point elevation; minor advantages are obtained by solutions with a high latent heat of vaporisation and low specific heat capacity. The first two requirements could appear counter-intuitive, since they are detrimental in the case of distillation processes per se. Quite surprisingly, the electrochemical parameters play a minor role in determining the efficiency. Our results allow to devise solutions for single-effect processes that give a high efficiency, close to the Carnot limit, for very low temperature heat sources (e.g. 11% for a temperature difference of 40 K), competitive with the more traditional techniques but much cheaper and easily down-scalable. ∗Università degli Studi di Milano, Dipartimento di Matematica, via Saldini 50, 20133 Milano (Italy). E-Mail:dbrogioli@gmail.com